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Chapter 8

Chapter 8. Harvesting Energy: Glycolysis and Cellular Respiration. Photosynthesis Provides the Energy Released by Glycolysis and Cellular Respiration. energy from sunlight. photosynthesis. 6 CO 2 + 6 H 2 O + light energy  C 6 H 12 O 6 + 6 O 2. C 6 H 12 O 6. CO 2. H 2 O. O 2. 6. 6.

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Chapter 8

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  1. Chapter 8 • Harvesting Energy: Glycolysis and Cellular Respiration

  2. Photosynthesis Provides the Energy Released by Glycolysis and Cellular Respiration energy from sunlight photosynthesis 6 CO2 + 6 H2O + light energy  C6H12O6 + 6 O2 C6H12O6 CO2 H2O O2 6 6 6 cellular respiration glycolysis ATP Fig. 8-1

  3. How Do Cells Obtain Energy? • An overview of glucose breakdown • The overall equation for the complete breakdown of glucose is C6H12O6 + 6 O2 6 CO2 + 6 H2O + ATP + heat

  4. How Do Cells Obtain Energy? • An overview of Glycolysis • The first stage of glucose breakdown is glycolysis • Splitting of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon sugar) • Two ATP molecules are produced • Glycolysis proceeds in the same way under aerobic (with oxygen) or anaerobic (without oxygen) conditions • Glycolysis occurs in the cytoplasm

  5. How Do Cells Obtain Energy? • An overview of Cellular Respiration • The second stage of glucose breakdown is aerobic respiration and occurs when oxygen is available • Two pyruvate molecules are broken down into six carbon dioxide molecules and six water molecules • For every two pyruvate molecules, an additional 34 or 36 ATP molecules are generated • Cellular respiration occurs in mitochondria

  6. How Do Cells Obtain Energy? • An overview of Aerobic fermentation • If oxygen is not available, the second stage of glucose breakdown is fermentation • Fermentation does not produce any ATP • Pyruvate remains in the cytoplasm and is converted into lactate or ethanol + CO2

  7. A Summary of Glucose Breakdown (cytoplasmic fluid) glucose glycolysis 2 ATP lactate fermentation 2 pyruvate ethanol + CO2 If O2 is available If no O2 is available 6 O2 34 or 36 cellular respiration ATP CO2 6 H2O 6 mitochondrion Fig. 8-2

  8. What Happens During Glycolysis? • Glucose activation • A glucose molecule is activated when it receives two phosphates from two ATPs, becoming fructose bisphosphate • Two ATPs are converted into two low-energy adenosine diphosphate (ADP) molecules

  9. What Happens During Glycolysis? • Energy harvesting • The six-carbon fructose bisphosphate is split into two, three-carbon molecules of glyceraldehyde-3-phosphate (G3P) • In a series of reactions, each of the two G3P molecules is converted into a pyruvate, generating two ATPs per conversion, for a total of four ATPs • As each G3P is converted to pyruvate, two high-energy electrons and a hydrogen ion are added to an “empty” electron-carrier nicotinamide adenine dinucleotide(NAD+) to make the high-energy electron-carrier molecule NADH

  10. The Essentials of Glycolysis • Summary of Glycolysis • Each molecule of glucose is broken down to two molecules of pyruvate • A net of two ATP molecules and two NADH are produced 4 4 2 ATP ATP 2 ADP ADP 2 2 C C C C C C C C C C C C C C C C C C G3P pyruvate glucose P P P fructose bisphosphate NAD+ 2 2 NADH Glucose activation Energy harvest 1 2 Fig. 8-3

  11. What Happens During Cellular Respiration? • Cellular respiration in eukaryotic cells occurs in mitochondria in three stages • Pyruvate is broken down in the mitochondrial matrix, • Keep in mind that each glucose molecule produces two pyruvate molecules • High-energy electrons travel through the electron transport chain • ATP is generated by chemiosmosis matrix inner membrane intermembrane space outer membrane

  12. What Happens During Cellular Respiration? • Mitochondria matrix reactions • The formation of acetyl CoA • Pyruvate is split, forming an acetyl group and releasing CO2 • The acetyl group reacts with Coenzyme A, forming acetyl CoA • During this reaction, two high-energy electrons and a hydrogen ion are transferred to NAD+, forming NADH

  13. What Happens During Cellular Respiration? • Mitochondria Matrix reactions • The Krebs cycle / Citric Acid cycle • Acetyl CoA is combined with a four-carbon molecule to form six-carbon citrate, and coenzyme A is released • Enzymes in the matrix break down the acetyl group, releasing two CO2 molecules and regenerating the four-carbon molecule for use in future cycles • Each acetyl group produces one ATP, three NADH, and one FADH2

  14. Reactions in the Mitochondrial Matrix Formation of acetyl CoA 1 3 NADH 3 NAD+ FAD FADH2 C CO2 coenzyme A coenzyme A C C CoA – Krebs cycle 2 C CO2 2 C C C acetyl CoA pyruvate NADH NAD+ ADP Fig. 8-5 ATP

  15. What Happens During Cellular Respiration? • Membrane reactions • Electron transport chain (ETC) • 10 NADH and 2 FADH for one glucose molecule • These high-energy electrons jump from molecule to molecule in the ETC, losing small amounts of energy at each step • The energy-depleted electrons are transferred to oxygen, which acts as a final electron acceptor • Energy-depleted electrons, oxygen, and hydrogen ions combine to form water

  16. What Happens During Cellular Respiration? • Chemiosmosis • This energy is harnessed to pump H+ into the intermembrane space, producing a high concentration of H+ • The energy is then captured in the bonds of ATP as H+ flows down its gradient • The flow of H+ through the synthase channel provides the energy to synthesize 32 or 34 molecules of ATP for each molecule of glucose (matrix) ADP ATP  H+ P 3 4 1 1 O2 H2O 2 2 H+ e–   2 e– FADH2 2 NADH e– 2 NAD+ FAD ATP synthase (inner membrane) ETC H+ H+ 2 H+ H+ H+ H+ (intermembrane space)

  17. Fig. 8-7 Summary of Cellular Respiration in Eukaryotic cells glucose (cytoplasmic fluid) glycolysis 2 NADH 2 ATP 2 pyruvate mitochondrion CoA 2 NADH 2 CO2 2 acetyl CoA Lipids and proteins can also be used by converting them to pyruvate or acetyl CoA 6 NADH Krebs cycle ATP 2 FADH2 2 4 CO2 H2O O2 32 or 34 electron transport chain ATP total: 36 or 38 ATP

  18. What Happens During Fermentation? • Why is anaerobic fermentation necessary? • For glycolysis to continue, the NAD+ used to generate NADH must constantly be regenerated • Under anaerobic conditions, with no oxygen to allow the ETC to function, the cell must regenerate the NAD+ for glycolysis using fermentation in cytoplasm • If the supply of NAD+ were to be exhausted, glycolysis would stop, energy production would cease, and the organism would rapidly die

  19. What Happens During Fermentation? • Why is fermentation necessary? • Organisms use one of two types of fermentation to regenerate NAD+ • Lactic acid fermentation produces lactic acid from pyruvate • Alcohol fermentation generates alcohol and CO2 from pyruvate

  20. What Happens During Fermentation? e n r a e t • Some cells ferment pyruvate to form lactate • Active muscle cellsregenerate NAD+ by fermenting pyruvate to lactate, using electrons from NADH and hydrogen ions • A variety of microorganisms that lack mitochondria, including the bacteria that convert milk into yogurt, sour cream, and cheese g i o e r n NAD+ 2 2 2 2 NADH NADH NAD+ 2 C C C 2 C C C C C C C C C (glycolysis) (fermentation) glucose pyruvate lactate 2 ATP 2 ADP Fig. 8-8

  21. What Happens During Fermentation? e n r a e t g i o e • Some cells ferment pyruvate to form alcohol and carbon dioxide • Many microorganisms, such as yeast • During alcohol fermentation, H+ and electrons from NADH are used to convert pyruvate into ethanol and CO2; this releases NAD+, which can accept more high-energy electrons during glycolysis r n 2 2 2 2 NAD+ NADH NADH NAD+ 2 C C C C C C C C C 2 C C + 2 C (glycolysis) (fermentation) pyruvate ethanol CO2 glucose 2 ATP 2 ADP

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